The results of the study, a summary of which was released by the Marine Corps this week, could factor into Pentagon deliberations about which roles, if any, should remain off-limits to women. The U.S. military services will soon submit their recommendations to Defense Secretary Ash Carter on the matter.

But Navy Secretary Ray Mabus has already publicly criticized the study. Mabus told National Public Radio he thought it was flawed, in part because of the mindset of the volunteers who participated.

“It started out with a fairly large component of the men thinking this is not a good idea and women will never be able to do this. When you start out with that mindset you’re almost presupposing the outcome,” Mabus told National Public Radio, in an interview aired on Friday.

The Marine Corps conducted the study using roughly 400 Marines, about 100 of them women, who volunteered to join the experimental Ground Combat Element Integrated Task Force, which was established for the study in July 2014 and wrapped up its work in July 2015.

Paul Johnson, a civilian in the Marine Corps who led the study, told Reuters in an interview the goal was to replicate real-life activities of young Marines in ground combat roles. The question for his research team was: Did integrating men and women make a difference?

“The answer is: yes,” Johnson said, saying that in 93 out of the 134 tasks analyzed, all-male units performed better.

The findings showed all-male squads moved faster, more so when carrying heavier loads. Women were more likely to sustain injuries, an executive summary of the study sent to Reuters said. The Marine Corps has not yet released the full study.

Mabus, an advocate for opening combat roles to women, told NPR that outside analysis had shown there were ways to ensure that women and men meet the same rigorous standards, pointing to one study by the Center for Naval Analyses.

He also said that the Marine Corps experiment likely should have set a “higher bar” for some of the women volunteers to participate.

Asked whether setting standards would eliminate some of the gaps shown in the experiment, Mabus said: “Everybody will have to be at the same level.”

“They will have to meet these standards that have now been set that simply didn’t exist before. Once you do that, there shouldn’t be these gaps,” he said.

Johnson said the study was set up to be objective, saying his team “didn’t have a preconceived notion when we walked into the experiment.”

Two years ago the U.S. services were told to develop gender-neutral standards for all jobs and to report by this autumn any jobs which should remain closed to women. A decision is expected in the coming months.

The release of the Marine Corps study came just a week after the U.S. Army announced it would open its elite Ranger School to all soldiers, regardless of gender. Last month, two women made history by becoming the first to pass the grueling leadership course.